The present invention concerns a rotating device which is capable of separating small solid or fluid particles having a cross-sectional dimension of about 5 to about 0.1 .mu.m, from gases.
Due to the action of centrifugal force, particulate material dispensed in a gas which is brought into rotation when passed through a separation channel of a centrifugal separator moves radially away from the axis of rotation towards a radial boundary which extends parallel to the axis of rotation and which forms the outer boundary of the separation channel in the centrifugal separator. The outer boundary of the separation channel serves as a means for collecting those particles which are able to reach and settle at this boundary and which can subsequently be removed from the gas flowing parallel along the collecting boundary.
The resultant velocity with which particles move radially can be assessed from a balance of the centrifugal force acting on the particles and the opposing drag force executed by the gas on the particle as a result of relative motion between the particles and the gas. The extent to which particles can reach the collecting boundary can be assessed from the time required for a particle to reach the collecting boundary in combination with the residence time, i.e., the time for the gas to pass through the separation channel.
Employing centrifugal accelerations of the order a thousand times the earth gravitational acceleration, one may calculate that particles dispensed in air and having a diameter of about 5 .mu.m and specific mass of about 2000 kilograms per cubic metre will move radially with a velocity of the order of about 1 metre per second. Assuming an axial velocity of the carrier gas of 5 meters per second, all such particles are able to reach the outer boundary when the axial length of the separation channel is of the order of five times the radial width of the separation channel. This corresponds to the dimensions of separation channels of commonly proposed centrifugal separators (US-A-4,231,771).
However, in order to separate particles in the micron and submicron range, a number of special requirements must be be met; requirements which are not satisfied by previously proposed centrifugal separators. To make this apparent, it is noted that centrifugally induced radial velocities decrease considerably with diameter of the particles. For particles with diameters of 1 and 0.1 .mu.m, respectively, and other conditions equal to those indicated above, one may calculate centrifugally induced radial migration velocities as small as 0.05 and 0.0005 m/s, respectively. Maintaining an axial velocity of the carrier gas of 5 meters per second, such particles are able to reach the outer boundary when the axial extent of the separation channel is as large as a hundred times and ten thousand times the radial width of the channel, respectively.
The ratio of axial length to radial width which is necessary to collect micron-sized and submicron-sized particles can be reduced by increasing the centrifugal acceleration and decreasing the gas velocity, but the change in these parameters is limited because of increased pressure loss over the separator and reduced throughput per unit area. Centrifugal separators aimed at economically separating micron-sized and submicron-sized particles from gases involve radial migration velocities which are considerably less than the axial throughput velocity, and axially extending separation channels which are considerably longer than their radial width.
As centrifugally induced radial velocities of micron-sized and submicron-sized particles are very small, small perturbations in the flow of the carrier gas may disturb the process of migration of such particles towards the outer boundary and prevent such particles from settling down at this boundary. A flow situation necessary for separating micron-sized and submicron-sized particles is obtained when the gas passes through the separation channel in a laminar flow. Laminar flow is generally achieved when the Reynolds number of the flow through the channel is smaller than 2400, preferably smaller than 2300. The Reynolds number is given by ##EQU1## where w.sub.o is the average axial velocity of the gas flow through the channel, v is the kinematic viscosity of the carrier gas and d.sub.H is the hydraulic diameter of the channel. The hydraulic diameter follows from the equation EQU d.sub.H =4 A/S,
where A is the cross-sectional area through which the gas flows and S the length of the curve enclosing the cross-section. For a circular pipe, d.sub.H is equal to the diameter. For a channel formed by narrowly spaced annular plates, d.sub.H is equal to twice the distance between the plates.
A typical value for the kinematic viscosity of gases is that of air which is approximately equal to 1.8.times.10.sup.-5 meter squared per second. Assuming an average axial velocity of 5 meters per second, a state of laminar flow can be obtained by employing a hydraulic diameter of less than 8 millimetres. In the case of a separation channel consisting of a circular pipe, this would require the diameter of the pipe to be less than 8 millimetres. In the case of a separation channel which is constituted of annular walls, under the quoted conditions the distance between the walls should be less than 4 millimetres.
Generally, if the Reynolds number is larger than 2400, the flow is no longer stable due to perturbations. Experiments have shown that secondary flows develop which can involve radial fluctuating velocities as large as one tenth of the axial throughput velocity and which are detremental to the process of migration and settling of micron-sized and submicron-sized particles. Centrifugal separators aimed at collecting particles in the micron and submicron range require separation channels of small radial width such that laminar flow exists.
Another factor causing secondary flow which can affect the separation process is rotation of the gas which is different from that of the surrounding boundaries of the separation channel. Differential rotation can be reduced by applying separation channels whose cross-sections are bounded by single curves. In the case of annular cylinders, this involves adding at least one azimuthal partition wall within the annulus, thus preventing the flow from making an entire swirl around the axis of symmetry of the device. The small radial width of separation channels having singly-connected cross sections, which is pertinent to laminar flow, ensure that differential rotations of the gas at the entrance of the separation channel are small and that these differential rotations decay smoothly with distance from the entrance and do not affect the migration and settling of small particles.
As the cross-sectional dimension of a laminar flow channel is small, the amount of gas which can be stripped from particles in such a channel is limited. Combining a large number of channels in one rotating unit provides a means for handling large amounts of gas to be cleaned. The channels can be dimensioned such that the previously identified conditions for separating small particles are satisfied. The large number of axially extending separation channels allows large amounts of throughput. Furthermore the separator of the invention is limited in size and is simple in design.
In the present invention account has been taken of the fore stated conditions necessary for separating small solid or fluid particles having a cross-sectional dimension of 5 micron to 0.1 .mu.m from gases using centrifugation. The invention relates to a rotational separator for separating particulate material from gas, comprising: a housing having a gas inlet, a gas outlet, and an outlet for separated particulate material; a centrifuge mounted for rotation in the housing and comprising separation channels which extend parallel to a common rotation axis; and means for rotating the centrifuge, characterized in that the particulate material separated has a cross-sectional dimension from about 0.1 to about 5 .mu.m, that the separation channels are provided with singly-connected cross sections over a substantial part of the axial separation channel length, and that the hydraulic diameter of the separation channels and the average axial gas velocity are selected in mutual dependence such that the Reynolds number is less than 2400 and the gas passes through the separation channels in a laminar flow. Preferably the Reynolds number is less than 2300.
In a first practical embodiment of the separator of the invention, the separation channels consist of at least one hundred, preferably one thousand and more circular pipes, or channels with non-circular cross-section, extending axially and arranged parallel to the rotation axis.
In a second practical embodiment of the separator of the invention the centrifuge comprises a massive cylinder perforated by at least hundred, preferably one thousand and more axially extending channels arranged parallel to the rotation axis.
In a third practical embodiment of the separator of the invention the centrifuge comprises at least ten, preferably 30 and more narrowly spaced concentric cylinders and each annulus between two adjacent cylinders is divided by at least one azimuthally placed, axially extending partition.
Solid particles which settle at the outer collecting boundary of a separation channel are subjected to shear forces executed by the gas, such forces increasing with decreasing width of the channel through which the gas flows. Due to continuous application of the centrifugal force, however, particles are subjected to wall friction which is larger than the gas friction, thus preventing reentrainment of particles in the gas flow. Separated particles can be removed by stopping the rotation of the channels, placing these vertically and employing the action of gravity or washing. In case of coagulation to the walls release of particles from the wall can be enhanced by mechanical or acoustic vibration.
In the case of fluid particles or droplets, a fluid film develops at the collecting boundary of the separation channel. If the rotating device is installed with its axis of rotation vertically, collected fluid flows downwards as a result of gravity, and due to the centrifugal force, is propelled outwards when leaving the separation channel. In the housing means can be employed for continuous transportation of the separated fluid.
Fluid which is propelled outwards when leaving any separation channel may enter separation channels which are located at larger radii, this being due to the axial component of the drag force executed on the particles by the gas, assuming that the gas enters the separation channel at the same end as where the collected fluid leaves it. To counteract such re-entrainment, one may extend the length of any separation channel such that its fluid exit and gas inlet extends beyond that of separation channels at larger radii.
Application of collecting walls which are placed nonparallel to the axis of rotation results in a component of the centrifugal force which acts parallel to the collecting boundary and can serve as a means for or to enhance the continuous transportation of collected particles along the wall. In the present invention application of such inclined walls is limited as these walls cause secondary flows, in particular under laminar flow conditions, such secondary flows being due to Coriolis forces and being able to disturb the process of radial migration and settling of micron-sized and submicron-sized particles. Expressed in radians, angles of inclination are limited to values which are of the same order of magnitude as the ratio of the radial width of the separation channel to its axial length.
Collection and transportation of solid particles can be enhanced by spraying a fluid or mist upstream of the separation channels. Particles can then be transported via the fluid film which develops at the collecting boundaries.
To bring the gas in rotation and to minimise pressure losses over the device, rotary means can be installed upstream and downstream of the separation channels. The rotary means can consist of volutes, stator blades, impellers, and/or mechanical drive.
Stator and rotor blades installed upstream and downstream of the separation channels can also serve as a means to control the division of the throughput over the various separation channels and to prevent internal circulations through the device whereby flows in opposite directions occur in separate channels. Control of the throughput over the separate channels can also occur by increasing the flow resistance over part of the separation channels using reduced cross-section, preferably at the downstream end of the separation channels.
Rotation of the gas upstream of the separation channels can be employed to separate larger particles prior to entrance in the separation channels. In this way, the amount and loading per unit time of the separation channels can be limited to those particles for which the channels are designed. In case of batchwise operation, this enables the time of operation until removal of collected particles to be increased.